PES Institute Of Technology Bangalore South Campus

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1 PES Institute Of Technology Bangalore South Campus (Hosur Road, 1KM before Electronic City, Bangalore ) Sem: V A and B Date: 27/8/1 Subject & Code: Operating Systems Marks: 50 Name of faculty: Y. R. Sudhakar Solutions for 1 st IA Test Dept: ISE 1. Draw a diagram to illustrate a view of Operating System Services. Write a note on any of the three services listed in the above diagram. (10 Marks) A view of Operating System Services Communications Processes may exchange information, on the same computer or between computers over a network Communications may be via shared memory or through message passing (packets moved by the OS) Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them Many types of resources - Some (such as CPU cycles, main memory, and file storage) may have special allocation code, others (such as I/O devices) may have general request and release code Accounting - To keep track of which users use how much and what kinds of computer resources

2 2. Explain the important principles relative to Operating System design & implementation; and the following OS design approaches. (i) Layered Approach (ii) Modular Approach (10 Marks) Following are the 2 important principles used in the design & implementation of OS. Policy determines what are the features to be supported by OS. Mechanism specifies how these features of the OS will be implemented. Layered Approach The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. layers are selected such that each uses functions (operations) and services of only lowerlevel layer Layered Approach Modular Approach Modular Approach Most modern operating systems implement kernel modules Uses object-oriented approach Each core component is separate Each talks to the others over known interfaces Each is loadable as needed within the kernel

3 3. (a) What are multi-processor-systems? Explain the different types of multi- processor systems. (5 Marks) Multiprocessor systems have 2 or more processors working together and sharing disks, I/O peripherals and main memory. In addition, has the following advantages. Increased throughput - Faster execution, but not 100% linear speedup. Economy of scale - Peripherals, disks, memory, shared among processors. Increased reliability Types of Multiprocessor Systems Based on role of Processors Asymmetric multiprocessing in which specific tasks of OS is assigned to different CPUs. A master processor controls the system resources and slave processors execute the user programs. An example OS is SunOS.1.3 (Solaris 1.x) Symmetric multiprocessing allows all the processors to execute all the tasks of the OS and be peers to each other. They share the main memory. An example OS is SunOS 5 (Solaris 2.x). Most modern Oses support SMP Based on Memory access model Uniform Memory access is the situation in which access to any RAM from any CPU takes the same amount of time. Non-uniform Memory Access (NUMA) is the situation in which a CPU accesses certain parts of the RAM much faster than other parts of the RAM. 3. (b). What is a virtual machine? Explain the just-in-time(jit) compiler used in JAVA virtual machine. (5 Marks) The fundamental idea behind a virtual machine is to abstract the hardware of a single computer (the CPU, memory, disk drives, network interface cards, and so forth) into several different execution environments, thereby creating the illusion that each separate execution environment is running its own private computer called a Virtual Machine. just-in-time (JIT) compiler can be used instead of JAVA interpreter in JAVA Virtual Machine to make the Java program execution faster. Here, the first time a Java method is invoked, the bytecodes for the method are turned into native machine language for the host system. These operations are then cached so that subsequent invocations of a method are performed using the native machine instructions and the bytecode operations need not be interpreted all over again.

4 . Consider the following set of processes. (10 Marks) Process Arrival Time Burst Time P1 0 1 P2 1 9 P3 2 1 P 3 9 (i) (ii) (iii) Draw Gantt charts showing the execution of these processes using FCFS, pre-emptive SJF, non-preemptive SJF and RR (Quantum = 1) scheduling schemes. Compute the turn-around time and waiting time for each process for each of the schemes above.. Compute the average turn-around time and average waiting time in each scheme and thus find the best scheme in this particular case. ) Waiting Time of a process is the total time spent in the ready queue waiting to get CPU time. Turnaround Time of a process = (time instant when process completes execution completion time instant when process arrives into the ready queue) Process Arrival timeline P1 P2 P3 P > t 0ms 1ms 2ms 3ms ms 5ms FCFS Gantt Chart P1 P2 P3 P 0 1ms 10ms 11ms 20ms P1 W1 = (0 0) = 0ms T1 = (1 0) = 1ms ( ) P2 W2 = (1 1) = 0ms T2 = (10 1) = 9ms Avg W = = ms P3 W3 = (10 2) = 8ms T3 = (11 2) = 9ms P W = (11-3) = 8ms T = (20 3) = 17ms ( ) Avg T = = 9ms

5 . Continued Process Arrival timeline P1 P2 P3 P > t 0ms 1ms 2ms 3ms ms 5ms Non-preemptive SJF Gantt Chart P1 P2 P3 P 0 1ms 10ms 11ms 20ms P1 W1 = 0 0 = 0ms T1 = (1 0) = 1ms ( ) P2 W2 = 1 1 = 0ms T2 = (10 1) = 9ms Avg W = = ms P3 W3 = (10 2) = 8ms T3 = (11 2) = 9ms P W = (11-3) = 8ms T = (20 3) = 17ms ( ) Avg T = = 9ms Preemptive SJF Gantt Chart (This is the best scheduling algorithm w/ least turn-around time for this process mix) P1 P2 P3 P2 P 0 1ms 2ms 3ms 11ms 20ms P1 W1 = 0ms T1 = (1-0) = 1ms ( ) AvgW = = 2.25ms P2 W2=(1 1)+(3-2) = 1ms T2 = (11-1) = 10ms P3 W3 = (3-3) = 0ms T3 = (3-2) = 1ms P W = (11-3) = 8ms T = (20 3) = 17ms ( ) Avg T = = 7.25ms RR (w/ Quantum = 1) Gantt Chart (At t =2, P2 goes to the front of Ready Q. At t=3, P gets behind P2 in Ready Q) P1 P2 P3 P2 P P2 P P2 P P2 P P2 P P2 P P2 P P2 P P ms 20ms P1 P2 W1 = 0ms W2 = x 1 = 8ms T1 = (1-0) = 1ms T2 = (18 1) = 17ms ( ) Avg W = =3.75ms P3 W3 = (3-3) = 0ms T3 = (3-2) = 1ms ( ) Avg T = = 9ms P W = 8 x 1 =8ms T = (20 3) = 17ms

6 5. Describe the process state diagram and process control block of a process. (10 Marks) As a process executes, it changes state new: The process is being created running: Instructions are being executed waiting: The process is waiting for some event to occur ready: The process is waiting to be assigned to a processor terminated: The process has finished execution Process State Diagram Process Control Block (PCB) Information associated with each process in execution is represented by a data structure in the kernel memory called PCB. Some of the following information about the process is stored in PCB. Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information

7 6. (a) Which scheduling algorithm, as an operating systems designer, would you implement. (0 Marks) There is no one scheduling algorithm that can be used for all possible process mix with different timing requirements. There are always trade offs and compromises. No marks will be awarded for saying shortest job first (SJF) would be implemented (as the burst time of processes in time-shared system are not known ahead of time). But an algorithm that estimates the burst time, so that SJF can be partially emulated would get 2 marks. Multi-level feedback queue scheduling would be a good choice. By varying the parameters to this algorithm, it is possible to emulate all the other algorithms we considered. Even this is not ideal as vast amounts of testing and guesswork would still be needed. This algorithm gives the flexibility to try various algorithms. Many other answers are possible and justification for the choice determines the marks awarded to this question. 6. (b) Daw the tree of processes created and mention the total number of processes created by the following program. P m #include <stdio.h> int main() P m1 P m2 P m3 { fork(); P m11 P m12 P m21 fork(); fork(); P m111 8 processes are created by this program } return 0;

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